Thermionic tube



Udk. 3(1), 193% TERMAN 1,978,918

' THERMIONIC TUBE Filed June 22. 1931 2 Sheets-Sheet l INVENTOR,

FREDER/ 5. TERM/1N. B .W

ATTORNEY Oct. 30, 1934.

F. E. T ERMAN THERMIONIC TUBE Filed June 22. 1931 2 Sheets-Sheet 2 INVENTOR, FREDERICK E. TERM/1N.

ATTORNEY.

Patented Oct. 30, 1934 UNITED STATES PATENT OFFICE 3 Claims.

My invention relates to thermionic vacuum tubes, and is particularly applicable to power tubes wherein the heating of the thermo-emis- I sive surfaces is responsible for a material power consumption.

Among the objects of my invention are: To provide a tube having a low internal impedance; to provide a tube wherein the energy which is usually wasted by radiation from the anode is utilized to provide at least a portion of the energy necessary for heating the cathode; and, in conformity with these'objects, to provide a tube wherein all of the elements of the tube may be operated at substantially the same temperature without interfering with the functioning of the tube.

Other objects of my invention will be apparent or will be specifically pointed out in the description forming a part of this specification, but I do not limit myself to the embodiment of my invention herein described, as various forms may be adopted within the scope of the claims.

Referring to the drawings:

Figure 1 is a'plan view of the active elements of a tube embodying my invention, the heat reflecting cover being omitted in order better to show the structure.

Figure 2 is an axial section-of the tube of Figure 1, the plane of projection being on the line 2-2 of the first figure.

Figure 3 is another axial sectional view, taken on the line 33 of Figure 1, showing the elements of the tube and, in addition, a portion of the tube stem including the lower portion of the envelope.

'35 The remainder of the envelope is omitted in this and the other figures in order to conserve space.

Figure 4 is a schematic diagram of another embodiment of my invention, the purpose of this figure being to show the combination of a large number of three-electrode tube elements, either in series or in cascade, within the same envelope.

Broadly considered, my invention comprises a vacuum tube having the usual cathode, anode, and usually, though not necessarily, grid or control electrode elements. Means are provided for retarding the escape of heat from these elements, which are preferably mounted concentrically. Retarding the escape of heat insures that all of the elements will operate at substantially the same temperature. The cathode is coated with some suitable thermoemissive substance, such as thorium or the alkaline earth oxides, which will emit electrons at a relatively low tempera- A ture. The other electrodes are not provided with 5 thermo-emissive surfaces, and moreover are preferably constructed of a material, such as chromium or chromium alloy, which definitely inhibits the emission of electrons even if provided with a coating which is ordinarily thermo-emissive. The outer member of the concentric assembly preferably is highly polished on both inner and outer surfaces in order to decrease, insofar as is possible, its capacity for radiating and absorbing heat. This outer member may be the plate of the tube, or there may be provided an additional outer reflector or otherwise insulating jacket which serves further to retain the heat within the assembly. 'I'hermo-emissive as here used, is defined as emissive of electrons in response to moderate temperatures.

The embodiment of my invention which I have chosen for detailed description comprises an envelope 1, of which only a fragment is shown, into which is sealed a stem 2 for supporting the leads 3, 5, 6 and 7.

Surrounding the stem is a band 8 to which are welded wire supports 10 which carry a cruciform frame 11 of refractory glass, fused quartz, or other suitable insulating material. A similar cruciform support 12 is mounted above the support 11, the two being joined by the wires which carry the tube elements, and forming therewith a single rigid frame. Proceeding concentrically outward from the axis of the tube, the elements shown comprise the filament 13, grid 14, plate 15, grid 16, filament 17, grid 18, and plate 20. The grids are supported by wires 21 and 22, extending vertically between the supports 11 and 12, the wires 22 being prolonged through the support 11 and welded to a wire 23 which makes contact with the grid lead 6, while the wires 21 terminate within the supports 11 and 12.

It will be noted that all of the grid support wires are secured to one pair of diametrically opposite arms of the support. The two anodes or plates 15 and 20 are welded to short wires 25 and 26 which terminate in the other pairs of arms of the cruciform supports. The two plates contact through the leads 27 and 28 and the plate lead 7 extending through the stem.

Extending through the center of the cruciform support 11 is an anchor 30 which connects with the filament lead 3, and to which is secured a system H of equally spaced radiating filament hooks 31. The anchor 32, passing through the center of the cruciform support 12, carries a similar system of filament hooks 31, and is electrically continuous with a lead 33 which passes over the top of the support 12, and outside of the plate 20 down to the stem, where it contacts with the filament lead 5. The filament 13 is stretched directly between the two anchors. The filament 17 is zigzagged back and forth between the upper filament hooks 31 and the lower filament hooks 31, all of the legs of the filament 13 and 1'7 being connected in parallel in the example here shown.

Surrounding the plate 20 is an outer jacket or cylinder 35, whose inner and outer surfaces are both preferably highly polished. This cylinder serves as a reflector to receive heat radiated from the plate and return it to within the tube. An additional reflector, comprising a disk 36, is preferably mounted to the top of the jacket 35 to prevent loss of heat by radiation axially along the tube. In order better to retain the heat within the tube it is also desirable that the plate 20 be itself of highly polished reflecting material.

The cathode surfaces are preferably coated with barium, strontium, thorium, or other material which is highly emissive of electrons at relatively low temperatures. The anode and grid structures are, on the contrary, preferably formed of material which does not itself readily emit electrons and which tends toprevent the emission of electrons from any ordinarily emissive surface which may be deposited thereon. Such a material is chromium or a chromium alloy of the type which has been marketed under the name Nichrome.

It will be seen that the combination of the cathode 13, grid 14, and plate 15 is of the ordinary structure of a triode tube. The combination of the filament 17, grid 16 and plate 15 also involve the proper relationships to give triode action, although this combination is inverted as compared to the usual structure, the cathode bei'ng'arranged on the outside, while the anode or plate is on the inside. Moreover, the plate 15 erves as an anode for both the filament 13 and the filamentl'l, the net eifect being the same as would obtain for two tubes in parallel. Still a third combination in ordinary triode relationship, is the filament 17, grid 18, and plate 20, these elements also being in parallel relationship to the other triode combinations already described. j

Owing to the fact that the tube is evacuated, there is substantially no heat lost from the variouselements by convection. The method of mounting the tube elements, and the fact that the connecting wires are preferably chromium alloys of high thermal resistance, makes the loss of heat from the tube by conduction extremely small. Radiation from the tube is also limited, although not, of course, entirely suppressed, by the reflecting surfaces of the anode 20 and the jacket 35 and reflector 36. The tendency therefore is for all of the elements within thetube to arrive at a common equilibrium temperature.

Due to the, nature of the materials of which the tube is constructed, there is rather a wide range of temperatures for all of the cathode elements will emit electrons while the grids and anodes will not, thus permitting the operation "of the tube in the usual triode manner even though it does not include the usual hot electrode and cold electrode. e

The heat insulating arrangement is, of course,

effective inconnection with the heat generated at the anodes as well as that from the filament.

In services where the load upon the tube is substantially constant, as is the case in oscillator tubes, it is possible with the tube of my invention to supply heat to the filament only until the anodes come up to temperature, after which all of the energy required for thermal emission is supplied from the anode. In other circumstances it is possible greatly to reduce the cathode current after the tube comes up to temperature, even though this current may not be discontinued entirely.

Owing to the very large surface of the anodes and cathodes, and the fact that the cathodes are so positioned that the electrons are rapidly drawn away therefrom, the anode circuit resistance of the tube as a whole is very low.

The principles described in detail above may, of course, be modified as is necessary for the service for which any specific tube is designed. Thus additional heat insulating means may be supplied in the form of additional reflecting jackets surrounding the tube elements. A further extension of the fundamental idea of my invention is shown in purely diagrammatic form in Figure 4,

wherein the envelope is indicated in a fragmentary manner as designated by the reference character 40. Mounted concentrically within the envelope are a plurality of anodes 41, grids 42, and cathodes 43. One or more heat insulating jackets surround the active elements of the tube, which may be all connected in parallel as in the tube first described, or the successive plates and grids may be insulated from each other and the elements operated in cascade or cascadeparallel. j Y t It will be noted that the necessity of operating the anode at a lower temperature than the cathodehas, in the past, precluded the possibility of disposing the cathode around-the anode, since the interior electrodes all tend to assume an equilibrium temperature substantially-equal to or higher than the electrodes surrounding them. The external cathode tube has intrinsically a lower impedance than one in which the cathode is centrally located, and such inverted tubes may be made in accordance with this invention, either combined in the same structure with elements having their usual relationships, as shown here, or in a simple, three element form.

I claim:

1. A thermionic tube comprising a pair of substantially concentric cathodes, an anode interposed between said cathodes to receive thermionic emission from each, and control grids interpositioned between said cathodes and said anode to control said emission.

2. A thermionic tube comprising a pair of substantially concentric anodes, a cathode positioned withinthe inner anode, a cathode between said anodes, and means for controlling thermionic emission from each of said cathodesto each of said anodes. 7

3. A thermionic tube comprising a pair of substantially concentric anodes, a cathode positioned within the inner anode, a cathode between said anodes, and means for controlling thermionic emission from each of said cathodes to each of said anodes, said cathodes and anodes being thermo-emissive at different temperatures to permit effective operation of said tube when said cathode and said anodes approach the same temperature.

I V FREDERICK E. TERMAN. 

